Hurricane Formation Issues and
Projected Impacts of Global Warming

Issues Related to Hurricanes as Warm Core Lows

(a) Hurricane formation and intensification issues today

Hurricanes are warm core lows. Even though they form in the tropical regions of the world, hurricanes are merely surface thermal lows that form in an air column that, in the case of Cape Verde hurricanes, is warmer than the air columns north and south of it (Fig 1). Hurricanes also have characteristic environmental lapse rates with a slope that is distinctive (Fig.2).

Studies have shown that hurricanes form as weak warm core lows move westward over the Atlantic near the Cape Verde Islands where the ocean surface is not much warmer than the surface of Africa. Yet the warm core lows intensify rapidly to Tropical Storms there. This happens because although the temperature profiles are roughly the same for the warm core lows associated with easterly waves and those that intensify to Tropical Depressions and stronger, the dew point profiles are much different.

The lows that move over the eastern Atlantic have near saturated conditions in their lower parts due to the evaportation of water from the very warm ocean. Dew points approach the actual temperatures. Because of that, lofting air parcels cool at the wet adiabatic rate (due to release of latent heat, shown as a narrow red arrow in Fig. 2) rather than the dry adiabatic rate of 5.5F/1000 feet (shown as a narrow blue arrow in Fig. 2).

Because of this the mean temperature of the air columns actually increases when easterly waves move into the eastern Atlantic. As shown in Fig. 3, this temperature increase can be very significant. Because of it, as shown in Fig. 1, the central air column becomes much warmer, and the pressure rises at its top become very marked. This creates greater upper divergence, and an intensification of the surface low.

Figure 3: Schematic Diagram showing the increase in mean temperature of air columns associated with warm core lows as they move westward off the coast of Africa.

This explains why tropical lows intensify rapidly in the tropical eastern Atantic and the tropical eastern Pacific. It also explains why hurricanes like Katrina or typhoons like Haiyan rapidly intensified when they moved over slightly warmer ocean temperatures. The warmer ocean temperatures were less significant to the intensification than was the large increase in dew points and, thus, amount of water vapor (and latent heat) that was processed by these storms.

(b) Hurricane Frequency and Intensity in the 21st Century

The factors governing hurricane formation and intensification are summarized above. How will Global Warming impact hurricane frequency and intensity in the coming 100 years?

First, although most warming will occur in the polar and subpolar areas, some warming will occur in the tropics. But it will occur evenly. So that the air columns shown in Fig. 1 will both be warmed. In essence, there will be no differential warming of air columns in the tropics due to Global Warming. This suggests that hurricane numbers, or frequencies, will not be impacted. However, modeling results are mixed, and some show an actual decrease of hurricane numbers and a few showing no change, and fewer showing a slight increase. On balance, one must conclude that current projections do not show a large change in hurricane frequencies.

Second, although the vertical temperature profile of hurricanes is not expected to change during the 21st century, the environmental lapse rate will be shifted towards warmer temperatures, as shown in Fig. 4.

If this temperature profile occurs near the Cape Verde Islands with warmer sea surface tempertures, the dew point temperatures will be nearly 3 C warmer than those today. Since for every 10 C rise in dew point temperatures, twice as much water vapor is present, this means that more latent heat will be released when air parcels are lofted. In that case, the surface cyclone would be much stronger.

In essence, projections are that more Category 3, 4, 5 hurricanes will occur. Although hurricane frequencies may not change much, more hurricanes will occur in the higher Safir-Simpson categories. This is the basis for the conclusions of the 2007 report from the Intergovernmental Panel on Climate Change (IPCC) as showin in Fig. 5.